WASHINGTON - Two Colorado congressmen sought assurances from a Trump administration official Thursday that forest firefighting measures are adequate, as a blaze in southwestern Colorado forced the evacuation of thousands of residents.

"When we're talking about being able to keep some of the firefighters safe, when you get into the rugged terrain in southwest Colorado and the San Juan Mountains, it's difficult to be able amble up and down those hillsides," Colorado U.S. Rep. Scott Tipton said during a congressional hearing. "Sometimes aerial firefighting is the best way to try to get some suppressional control over the fire."

New Colorado Springs fire restrictions issued

Instead, the Forest Service plans to operate with more "call-when-needed" air tankers that could be drawn from other organizations, including the U.S. military and the Canadian government.

Christiansen told him the Forest Service was using 25 large air tankers so far this year and planned to use 30 more later this summer.

"Large air tankers and aircraft are a very important wildfire response tool," Christiansen said. "But they alone don't put out the fires."

Firefighters on the ground also play an important role, she said.

"We work hard to find the right balance," Christiansen said.

In addition to protecting the environment, the Forest Service seeks to demonstrate "fiscal integrity to the taxpayers of this country," she said.

Tipton authored the Healthy Forest Management Act of 2015, which would have given forestry companies greater authority to trim forests and remove underbrush. In addition to creating forestry jobs, the bill sought to reduce potential fuel for wildfires.

Although the bill never won final approval in Congress, some of its concepts for trimming forests were merged into other legislation.

Christiansen said overgrowth of the forests was contributing to the wildfires, particularly when beetles infest trees and leave them as dry wood ready to burn.

"In my simple terms, I call it a smorgasbord" for beetles, Christiansen said. With the current growth of forests, "We're overstocked," she said.

Until the mid-1990s, forestry companies harvested an average of 10-to-12 billion board feet of timber every year, according to a congressional report. By 2015, less than 2.9 billion board feet were harvested, largely because of complaints by environmentalists about stripping forests and removing habitat for wildlife.

At the same time, the number of wildfires increased. Last year, more than 8.5 million acres were burned by wildfires, the most since 2012.

Colorado U.S. Rep. Doug Lamborn, R-Colorado Springs, said he thought the Forest Service showed a "reluctance" to seek contracts for "very large air tankers," or VLATs, that could help fight wildfires.

Christiansen disagreed the Forest Service was neglecting its duties with aircraft.

"We don't always need 19,000 pounds of retardant," she said.

However, she also assured Lamborn the Forest Service would cooperate with Congress in ensuring the availability of the aircraft.

About the same time the congressional subcommittee was meeting Thursday, firefighters were telling hundreds of residents around Hermosa to prepare for evacuation. More than 1,000 homes already were evacuated.

The week-old blaze, known as the 416 Fire, has burned about 8 square miles of land 10 miles north of Durango. Firefighters plan to use fire lines to limit the spreading flames.

Despite use of air tankers, sprinkler systems and fire hoses, the wildfire is only a little more than 10 percent contained, according to officials.